Given that NADPH oxidase has been implicated in the modulation of the alternatively-activated macrophage pathway, and that NADPH oxidase components are differentially expressed after lipopolysaccharide (LPS) injection in COX-1-/- and in COX-2-/- mice, we hypothesized that genetic deletion of the functional p47phox subunit of NADPH oxidase will attenuate the inflammatory response after LPS via reduction of oxidative stress and activation of the anti-inflammatory alternatively-activated macrophages/microglia. We found that p47phox-/- mice had reduced glial activation after LPS, as demonstrated by reduced immunostaining of microglial and astrocytic markers. Neutrophil infiltration was assessed by 7/4 immunostaining and was reduced in p47phox-/- compared to WT mice. Brain levels of MPO, another marker of neutrophils, were also reduced in p47phox-/- mice. Gene expression of TNF-alpha, a marker of classical activation, was decreased after LPS in p47phox-/- mice compared to WT mice, whereas expression of chitinase 3-like 3 (Ym1), a marker of the alternatively activated macrophage pathway, was increased. These data suggest that oxidative stress is an important component of neuroinflammatory damage and that the NADPH oxidase may play a role in switching the macrophage/microglia phenotype from an alternative, anti-inflammatory to a classical pro-inflammatory state. Thus, inhibition of NADPH oxidase may represent a novel anti-inflammatory approach.